TT 3-15P vs Kahles 3.5-28

[Guns&WhiteWater]

Does anyone have any experience with both of these two optics? I understand the TT will have excellent IQ, and the Kahles should be good enough, but my main concern is eye box forgiveness, especially the minimum power and at 15 power for each.

Euro lists the TT’s exit pupil as being 11.5mm on its minimum magnification. Kahles lists theirs as having 7.4mm. Does this equate to the Tangent as having a more forgiving eye box, or is there more to this than the exit pupil?

 

[Glassaholic]

Euro lists the TT’s exit pupil as being 11.5mm on its minimum magnification. Kahles lists theirs as having 7.4mm. Does this equate to the Tangent as having a more forgiving eye box, or is there more to this than the exit pupil?

Yes, the erector and focal length also play a role in how forgiving the eyebox is. The TT is very forgiving and I'd have a hard time thinking the 8x Kahles K328i would be better; however, I have not seen the Kahles K328i so its hard to know what trickery they may have used to help improve eyebox performance, but in fact, you mention one of those "tricks" when you say Kahles lists their exit pupil at 7.4mm. In general exit pupil is mathematically equated by taking the objective size divided by the magnification so in theory the Kahles should have 14.3mm exit pupil at 3.5x, but one way to help with such a high erector is to limit the exit pupil which it appears Kahles has done here. Does a larger exit pupil equate to a more forgiving eyebox - in general yes, but there are other factors within the optical formula that effect eye box which is why eye box is not a spec that any manufacturer provides, you have to experience it for yourself. Finally, exit pupil is not always linear, so just because Kahles is limited at 3.5x doesn't mean it will be as limited at 15x, you would have to talk with Kahles and see if they'd be willing to share the exit pupil values at multiple magnifications.

That said, there are some guys on the Hide who have TT's, ZCO's et al and also have the K328i so hopefully one of them will chime in and offer up some real world thoughts as to how forgiving these two scopes are in comparison at 15x...

 

[Gunny89]

The TT 3-15P is likely to feel more forgiving overall, especially at mid to high magnifications. The Kahles 3.5-28i might have a trick up its sleeve at lower powers, but that doesn't necessarily mean it holds the advantage throughout its range